This invention relates to the manufacture of glyoxylic acid esters obtained by oxidation of the corresponding esters of glycolic acid and, more particularly, to an improved process for removing from said glyoxylic acid esters low concentrations of acetic acid other acids and hemiacetals formed in the manufacturing process as well as the efficient recovery of glycolic acid esters.
Polyacetal carboxylates have been demonstrated to be useful as builders in detergent formulations. Crutchfield U.S. Pat. No. 4,144,226 describes the preparation of polyacetal carboxylates by polymerization of an ester of glyoxylic acid, preferably methyl glyoxylate. The glyoxylic acid ester monomer may be prepared by vapor phase oxidation of the corresponding ester of glycolic acid. Side reactions occurring under the oxidation reaction conditions result in the contamination of the reaction product with water, an alkanol derived from the ester, and minor concentrations of acids such as acetic, formic, glycolic and glyoxylic as well as hemiacetals. To minimize the loss of yield to side reactions, the oxidation reaction is carried out with a deficiency of oxygen, so that the reaction mixture also contains a substantial fraction of unreacted glycolate ester.
In order to obtain a satisfactory yield and a high quality polyacetal carboxylate product from the polymerization reaction, it is necessary that the glyoxylate monomer be of high purity and that, in particular, it be purified to be substantially free of water, alkanol, carboxylic acids and unreacted glycolate ester. A typical process for recovering high quality glyoxylate ester is described in U.S. Pat. No. 4,502,923, wherein the product of the oxidation reaction is subjected to multiple distillation operations, first at low temperature under vacuum for removal of low boilers, primarily water and methanol, then at higher temperature under vacuum for removal of glycolate ester as an overhead stream, and finally at atmospheric pressure for removal of glyoxylate ester as an overhead stream. As indicated by an inflection in the vapor/liquid equilibrium curve, more glycolate ester can be removed from a mixture containing glyoxylate ester at low absolute pressure than at atmospheric pressure. The converse is true for glyoxylate ester. Bottoms from the glyoxylate atmospheric pressure distillation contain the glycolate that has not been removed as overhead in the glycolate vacuum still, as well as the hemiacetal of the glycolate and glyoxylate, and other high boilers. This stream is recycled to an earlier step in the process, typically the feed to the low boiler still.
Glyoxylate ester reacts with water to form the hydrate, and with both alkanol and glycolate to form the corresponding hemiacetals. These are equilibrium reactions which may proceed in either direction not only in the reaction step but also in the distillation steps and beyond. Although the first vacuum distillation step may be effective for removal of free water and alkanol, glyoxylate hydrate and glyoxylate/alkanol hemiacetal remain in the still bottoms and are carried forward to subsequent steps where they may dissociate to form additional free water and alkanol. Under the conditions of the atmospheric glyoxylate still, in particular, removal of glyoxylate ester from the liquid phase tends to promote the dissociation of hydrate and alkanol hemiacetal.
It has been discovered that upon incorporating into the process the various recycle streams necessary to provide an economic glyoxylate ester recovery system, acetic acid formed during the oxidation reaction accumulates in the system and preferentially exits with the glyoxylate ester. The resultant contamination of the glyoxylate ester causes a yield loss of methyl glyoxylate polymer since polymer endcaps formed by the acetic acid molecules are only temporary.
Furthermore, the accumulation of acetic acid increases the acidity of the system which in turn accelerates the autocatalytic decomposition of methyl glycolate and methyl glyoxylate, thereby reducing the recovery of those esters.